Certain natural fermentation products, notably mevinolin and compactin, and a number of semi-synthetic and totally synthetic analogs thereof are known to be inhibitors of HMG-CoA reductase and hence useful in the treatment of disease states resulting from elevated levels of serum cholesterol. In particular, U.S. Pat. No. 4,255,444 discloses compounds of formula: ##STR3## and the corresponding dihydroxy acids wherein Z is, inter alia, a naphthyl group either unsubstituted or substituted with one or more halogen atoms and/or C.sub.1 -C.sub.3 alkyl groups, preferably chlorine or methyl groups, especially 2-methyl-1-naphthyl, but also disclosing naphthyl and 2,6-dimethylnaphthyl.